I am requesting support for a five-year program of research on the nature of elementary cognitive processes. The major goal of this research is the refinement of a theory of mental activity. This theory is based on the premise that all mental activity is supported by mental structures which automatically evolve as a result of personal experience. As these structures evolve, the activity they support becomes increasingly automatic, involuntary, and unconscious. The theory is shown to provide a parsimonious account of many diverse phenomena observed in sensation, perception, decision, action, memory, and attention. Further development of the theory requires a systematic study of how structures evolve in response to experience. New experimental procedures and techniques of analysis are proposed to support this study. Finally, four programs of experimentation are described which explore (1) the way in which structures evolve, (2) the effects of expectation and context on the activation of structures, (3) the nature of fatigue and recovery in cognitive structures, and (4) the interactions of structures in attentional limitations.